This invention relates to an optical disc reproducing apparatus for reproducing a so-called compact disc interactive (CD-I) on which the video information, such as picture or letter information, computer data, application programs etc. are recorded besides the audio information.
There is currently proposed a system, known as a CD-I system, in which video data, such as natural picture, RGB graphics, color lookup table graphics, animation etc., computer data, or application programs, abbreviated hereinafter to programs, are recorded on a compact disc, in addition to audio data, and in which a user may use these data interactively. Inasmuch as this CD-I system has the functions of reproducing the voice, letters or pictures, and executing program(s), and may be used in connection with AV devices for mainly audio and video information and electronic publication mainly of letter or character information as well as educational or amusement devices, which rely upon program execution, it is thought to be a promising new type medium system.
The CD-I system is made up of an optical disc on which audio data, video data or the like are recorded, referred to bereinafter as a CD-I disc, and a player for reproducing a CD-I disc, referred to hereinafter as a CD-I player. Various specifications have been prescribed in connection with the CD-I system with a view to maintenance of interchangeability and extensive propagation for domestic use.
For example, in a CD-I disc, audio and video data, for which high accuracy is not a particular requirement, are recorded in accordance with a so-called form 2 of the CD-I data format, while computer data or programs, which are in need of error detection and correction, are recorded in accordance with a so-called form 1 of the CD-I data format.
With the CD-I system, four sound quality levels may be used, so that the audio data are processed with four-level data compression or coding, depending on the desired sound quality level, before being recorded on the CD-I disc. Thus the recorded data may be 16-bit PCM (Pulse Code Modulation) audio data for a conventional compact disc, 8- or 4-bit ADPCM (Adaptive Differential Pulse Code Modulation) audio data, or so-called A-level, B-level or C-level audio data.
On the other hand, four picture quality levels, namely the picture quality for natural pictures, e.g. color photographs, that for RGB graphics, that for color lookup table graphics (CLUT graphics) and that for animation, may be used with the CD-I system, so that the video data are processed with data compression or encoding in accordance with the desired picture quality before being recorded on the CD-I disc. That is, with the picture quality for natural picture, the video data are encoded with 8 bits per pixel and processed with non-interlaced DYUV encoding for recording. With the picture quality for RGB graphics, three color signals (RGB signals) for each pixel are encoded with 5 bits for recording, with the picture quality for CLUT graphics, the video data are recorded so that 256 colors may be displayed simultaneously by allocating 8 bits for each pixel. Finally, with the picture quality for animation, the video data for the CLUT graphics are processed with data compression by the so-called run-length coding for recording.
Meanwhile, a 68000 series microcomputer, which is a 16-bit CPU developed by MOTOROLA Inc. is preferentially employed as a microcomputer making up a CD-I player, while a so-called compact disc real-time operating system (CD-RTOS), corresponding to the existing OS-9, an operating system developed by MICROWEAR Inc. but added to by input/output managers and drivers for graphics/visual, pointer and audio processing operations, is preferentially employed as an operating system (OS), in consideration of program interchangeability and execution efficiency on the CD-I disc. In addition, an X-Y device, which may be accessed on the pixel-by-pixel basis, and a trigger button (for execute/stop), are preferentially employed as input devices. Besides, a compact disc (CD), on which only music has been recorded, may also be reproduced with the CD-I system.
The CD-I player, for which a variety of specifications are prescribed as depicted above and which may be used as a variety of devices, such as AV, electronic publishing, educational or amusement devices, are preferably so designed as to cope the speed of execution which is suited to the particular application or usage or which is desired by the user. For example, while the retrieval speed as fast as possible is desirable when the player is used as an electronic publishing device, such as that for dictionaries, it would be meaningless to increase the operating speed when the player is used as, for example, an AV device. Also, when the player is used as a game device, a user skilled in games would prefer a faster game progress, that is, a faster speed of execution, whereas an unskilled user would prefer a slower speed of execution.
Meanwhile, the speed of execution of a program depends on the frequency of the clocks supplied to the CPU, such that the speed of execution is increased with a higher frequency. The CD-I player in current use has a fixed clock frequency, so that the program can be executed only at a fixed speed.